This invention relates to the field of petroleum refining and more specifically to methods for the conversion of petroleum crudes, heavy hydrocarbon fractions, as well as petroleum residues by hydrogenation.
The charge used in the method of this invention can be any hydrocarbon oil of a high boiling point, for example, above 350.degree. C. The initial source of the oil can be, for example any petroleum crude, materials such as shale oils or oily sands, or liquid hydrocarbons resulting from coal liquefaction. The charge usually contains at least 1% asphaltenes, determined by precipitation with heptane.
Petroleum and petroleum fractions are very complex mixtures in which, besides hydrocarbons, there are various compounds containing principally sulfur, nitrogen, oxygen, as well as metals. The quantity and nature of these compounds vary according to the source of the crude from which they are derived and the fractions concerned. They are generally impurities detrimental to the good quality of petroleum products on account of contamination, corrosion, odor, instability. Of the numerous methods recommended for their removal, catalytic treatments in the presence of hydrogen are the most widespread.
This technique has the advantage of allowing products of good quality to be obtained from petroleum crudes and residues with a high content of impurities.
The difficulties in treating these charges are related principally to the presence of asphaltenes and metals which, under insufficiently controlled conditions, cause the deactivation of catalysts.
The contaminating Metal agents can be present as oxides or sulfides; usually, however, they are in the form of organo-metal compounds such as porphyrins and their derivatives. The most common metals are vanadium and nickel.
Asphaltenes occur in the form of a colloidal suspension which under hydrogenation refining conditions may agglomerate and become deposited on the catalytic composition; thus fixed-bed hydrotreatment of these charges does not give satisfactory results, the catalyst becoming deactivated following the deposition of coke and metals.
The boiling-bed technique, applied to heavy charges (FR Nos. 2,396,065 and 2,396,066), enables the consumption of catalyst to be reduced 1.5 times compared with the prior fixed-bed methods and the production of liquid products to be increased 2.5 times compared with other methods which involve prior deasphalting of the initial charge. This type of process converts the soluble organo-metal compounds well, but is less efficient as regards the asphaltenes. In addition, there is a certain erosion of plant and catalyst. One difficulty is the filtration of the product to separate from it the fine solid particles in suspension.
Another technique mitigating these disadvantages, as it gives the high molecular weight asphaltenes better accessibility to the catalytic sites, is disclosed in numerous patents such as FR No. 1,373,253 and U.S. Pat. No. 3,165,463.
With this objective in mind, catalytically active metal compounds are used in divided form. These are compounds of metals selected from groups IV, V, VI and/or the iron group, in suspension or in solution. During the hydrorefining treatment, they undergo several conversions and are finally present in finely divided dispersed form or as sludge, usually as sulfides.
This technique can have various names, such as hydrogenation, hydrotreatment, hydrorefining or hydroviscoreduction.
The technique requires that the heavy hydrocarbons and the catalyst sludge be separated from the total product leaving the reaction zone. This operation has so far been carried out by means such as centrifuging, filtration, distillation, followed by separation of the sludge containing the catalyst; the total or partial recycling of the latter has also been suggested for treating a fresh charge of hydrocarbons.
This sludge also contains, besides the metal used as catalytically active material, variable quantities of nickel, vanadium or other metals derived from the destructive conversion of asphaltenes. Before recycling, part of this sludge can be drawn off as a catalyst purge and replaced by a substantially equivalent quantity of fresh catalyst compound.
The principal difficulty in the techniques described above resides in the separation of the catalyst from the reaction product. Certain methods, in particular distillations, are costly in energy and require relatively high temperatures which the catalyst does not tolerate in the absence of hydrogen; other methods require major cooling of the effluent, which is not very advantageous since it then has to be reheated and part of it returned to the reactor; finally, other methods, such as filtration, result in a relatively rapid clogging of the filters.
U.S. Pat. No. 4,411,790 describes a tangential ultrafiltration method applied to spent lubricating oils or petroleum distillation residues, but this gives relatively unsatisfactory results as regards the filtration of the asphaltenes and the vanadium, since a large part of these passes through the inorganic membranes. In addition, the membranes clog relatively rapidly, in particular when the charge contains more than 8% by weight of asphaltenes (heptane test).